Early childhood developmental delay (DD) places young children and their families at elevated risk for a variety of problems. In addition to the various language, cognitive, and motor deficits that define young children failing to reach expected developmental milestones, young children with DD are at significant risk for serious behavior problems, and their families are at risk for very high levels of stress. Many affected children are from ethnic and racial minority backgrounds and are more likely to live in poverty and rely on government services, such as Early Intervention (EI) via Part C of the Individuals with Disabilities and Education Act (IDEA). Despite promise with EI, these services typically focus on the delay rather than associated behavior problems that are significantly and causally linked with functional and academic outcomes. Furthermore, EI services end at 3 years, and subsequent interventions can only be received through Part B of the IDEA through school-based special education. This gap in family-based services can be especially problematic for children living in poverty and from ethnic and racial minority groups for whom transportation and other barriers to services are considerable and clinic-based treatments may not be as feasible. Moreover, substantial numbers of affected families report children's services are too far away, and many indicate that they have no way to get to a clinic. Inadequate numbers of professionals trained in evidence-based programs impinge on the availability of care, as a considerable proportion of U.S. counties have no psychologist, psychiatrist, or social worker. Therefore, this new and early investigator-initiated R01 application proposes to build on extensive pilot work conducted by the investigators to evaluate, via a randomized controlled trial, the incremental utility of an Internet-delivered parent-training program for disruptive behavioral problems in traditionally underserved young children with DD. Families dwelling in rural or impoverished regions can participate in real-time treatment remotely, regardless of their geographic proximity to a clinic or ability to transport to services. Moreover, treating families within their natural settings (i.e., their homes) may enhance the ecological validity of care by providing live observation and feedback to parents in the very settings in which child behaviors are problematic. Specifically, we are interested in the impact of Internet-delivered Parent-Child Interaction Therapy (I-PCIT) on child disruptive behavior problems, parenting practices, parental distress, and early academic skills relative to traditional referrals as usual (RAU) for behavioral problems among youth aging out of Part C EI services. A secondary goal is to evaluate mechanisms that explain under which circumstances, for whom, and through which mediating pathways I-PCIT is most effective for young children with DD. Technological innovations overcome traditional barriers to effective care for traditionally underserved populations and are proposed to have a significant public health impact on the underserved population of young children and their families aging out of Part C EI services.